I couldn't find this answered in the forums, the TUM or the User Guide. If I missed it, please let me know.

I'm looking to make a custom map border and not sure I understand how the border works etc.

So as I get ready to map my mega dungeon I want to make sure I keep in mind what's needed to make print ready posters. So, I'm planning on 24x36" poster size. From what I've learned I need to make the map 24.5x36.5" for bleed and then leave room inside for trim accuracy issues (i.e. leaving about 23.8x35.8" usable).

That leaves me planning for 23.5 x 35.5 for the outside of the visible decorative border. The map border might be as wide as 1", leaving 21.5x34.5" of usable map area. Since I want about 1600 x 2100 ft usable map area, this gives me a printed scale of 80ft per inch. That gives me a CC map size of 1960x2920ft.

So, I create a custom map with no border of a size of 1960x2920. Then on the Map Border or Background sheet I can draw my border as I want? (And create a sheet background that goes all the way to the edges.

Is it that easy? What about the white frame? Do I need to worry about that or will it automatically be outside my 1960x2920?

It sounds a bit weird to me that you are targeting a spesific number of feets/inch like this. This makes sense when creating a miniature battle map, where the squares need to be a certain size to fit the miniatures, but at 80 feet/inch, you are just at a pretty map anyway, and since map size isn't directly linked to export size in any case, it seems a bit weird to be this spesific. I would rather go for making the map the way I want it to be, and then simply export an image with the correct size for the poster in the end. You'll need to know a ballpark number obviously, because this affects your detail level, but no need to that spesific imho.

So, I create a custom map with no border of a size of 1960x2920. Then on the Map Border or Background sheet I can draw my border as I want? (And create a sheet background that goes all the way to the edges.

Basically, yes. You'll want to put it on the MAP BORDER sheet, as BACKGROUND is usually below everything else, which is not what you want for your decorative border.Note that you can draw your decorative border both outside and inside the map dimensions you specified, but most drawing tools won't work outside of that area, only the basic tools. (You can always modify drawing tools to not stop at the map border though)

What about the white frame? Do I need to worry about that or will it automatically be outside my 1960x2920?

If the template you are using contains a screen, it will automatically be outside the map size you specify.

I get that being so specific on the printed scale is kind of strange. (And that working backwards from print size is not normal either.) But I already have an outline of the map so I know it's dimensions and I figured I might as well see if a "normal-ish" scale would work I might as well use it.

So the screen will be outside the defined edge of the map. that was what I was worried about.

Next question, I've read the various tutorials linked from the Cartographer's Guild on making a make border, but none of those are using CC and I couldn't find one on the forums here. Is there a technique to make a repetitive decorative border? I have an idea of what I want to do (dwarven runes or etchings) and the dungeon symbol in the corners that I've already designed. But I don't know if I should make it as a symbol or perhaps a bitmap and use it as a fill.... Any suggestions appreciated.

Drawing the map border is just like drawing anything else in CC3+. You can often get great results if you have a pattern you can use as a fill style, especially for the repetitive parts. If you have annual Vol 2 (2008), Issue 23 contains some tools and pointers for working with the map border.

Lots of people here make gorgeous map borders, so they may have more artistic tips for you. Also, many of the maps in the community atlas, as well as maps showcased in this forum have a nice map border you can use as inspiration.

I've sketched out about a dozen patterns and came up with the following. As you can see, I'm working on variations of it, but I do have one problem. Right now you can see the lines are linear paths, but when they touch themselves, they don't intersect well. I'm concerned when I export this and add effects it will not look good.

Do I need to convert the line to a poly or something to solve the overlap part?

Any feedback on the design is more than welcome.

Also, I'm thinking of making this into a symbol so it will be easier to re-use and to be able to make modifications to it in one place and have it reflected throughout the border.

The easiest way to deal with artifacts form lines overlapping itself is to split it into two lines. The SPLIT command splits a line into two distinct entities, but with no visible gap. Use this to split the line at a point along it such that each line in the intersection becomes a separate line. Also, for best results, use it in the middle of a line segment, not in a corner